As a mounting method in mounting a surface-mounted electronic part on a substrate or the like, a method of mounting an electronic part by soldering an external electrode of the electronic part to an electrode for mounting (land electrode) on the substrate is widely used.
As a solder paste used for such mounting by soldering, for example, a solder paste including a mixture of (a) a second metal (or an alloy) ball made of a high melting point metal such as Cu, Al, Au, or Ag or a high melting point alloy containing the high melting point metal, and (b) a first metal ball made of Sn or In is proposed (Patent Document 1).
Further, in Patent Document 1, a joining method using the solder paste, and a method for manufacturing an electronic equipment are disclosed.
By the way, when soldering is performed by using the solder paste described in Patent Document 1, as schematically shown in FIG. 6(a), the solder paste including low melting point metal (e.g., Sn) balls 51, high melting point metal (e.g., Cu) balls 52 and a flux 53 is heated to react, and after soldering, as shown in FIG. 6(b), a plurality of high melting point metal balls 52 are connected to one another with an intermetallic compound 54 formed between a low melting point metal originating from the low melting point metal ball and a high melting point metal originating from the high melting point metal ball interposed therebetween, and by this connecting body, joining objects are connected (soldered) to each other.
In the case of the solder paste described in Patent Document 1, the intermetallic compound between the high melting point metal (e.g., Cu) and the low melting point metal (e.g., Sn) is produced by heating the solder paste in THE soldering step, and in the combination of Cu (high melting point metal) and Sn (low melting point metal), a large amount of Sn being a low melting point metal remains because the diffusion rates of these metals are slow. In the case of a solder paste in which Sn remains, the joint strength under elevated temperatures is significantly deteriorated, and there may be cases where a product cannot be used depending on the type of the product to be joined. Further, there is a possibility that Sn remaining after the step of soldering may be melted and flowed out in the subsequent another soldering step, and there is a problem that this soldering is low in reliability as high temperature solder which is used for a bonding method with temperature hierarchy.
That is, for example, in a manufacturing process of a semiconductor device, when a semiconductor device is manufactured after undergoing a step of soldering, and then the semiconductor device is to be mounted on a substrate by a reflow soldering method, there is a possibility that Sn remaining after the step of soldering in the manufacturing process of a semiconductor device is melted and flowed out in the step of reflow soldering.
Further, it is necessary to heat the solder paste at a high temperature for a long time in the soldering step in order to convert the low melting point metal entirely to the intermetallic compound so that Sn may not remain, but this heating is practically impossible in consideration of the balance with productivity.
In order to solve the above-mentioned problems, there is proposed a solder paste including a metal component containing a first metal powder and a second metal powder having a higher melting point than the first metal powder, and a flux component, wherein the first metal is Sn or an alloy containing Sn, and the second metal (Cu—Mn or Cu—Ni) is a metal or an alloy which forms, with the first metal, an intermetallic compound exhibiting a melting point of 310° C. or higher, and has a lattice constant difference of 50% or more, the lattice constant difference being a difference between the lattice constant of the intermetallic compound produced first around the second metal powder and the lattice constant of the second metal component (Patent Document 2).
In addition, Patent Document 2 mentions Cu—Mn or Cu—Ni as the second metal.
Further, Patent Document 2 proposes a joining method and a joint structure that use the above-mentioned solder paste, and a method for manufacturing an electronic equipment.
It is described that in accordance with the joining method using the solder paste, joining by which the amount of remaining Sn is largely reduced to avoid the flow out of solder at the time of reflowing, and which is excellent in the joint strength and joint reliability at a high temperature can be performed.
However, in the case of the joining method using the solder paste of Patent Document 2, since a diffusion reaction of the second metal such as Cu—Mn or Cu—Ni with the first metal such as Sn or a Sn alloy rapidly occurs, the time during which Sn exhibits a liquid state is short and an intermetallic compound having a high melting temperature is formed soon, and therefore there is a possibility that air gaps are generated within the joint portion. Accordingly, a joining method in which joining having higher joint reliability can be performed is expected.
Patent Document 1: Japanese Patent Laid-open Publication No. 2002-254194
Patent Document 2: WO 2011/027659 A